This is Latin. Here's my best shot at transcribing it:
L[UCIO] FLAVIO L[UCI] FLAVI PAULI F[ILIO]
SER[VIO] LONGO DEC[EMVIRO] AEDIL[I ] CURAT
S[ACR]AE SANCTUARIAE QUAESTOR ET
[]AE []MILONICI F[ILIAE] MODESTAE UXORI
[] SUI FEL[ICITATI?] V S D[ONUM] D[EDIT] H C
Translation is difficult. Significant parts of the inscription are obliterated or otherwise illegible. A very fragmentary and tentative translation:
"To Lucius Flavius, son of Lucius Flavius Paulus ... to Servius Longus, Decemvir and Aedile ... (who) as treasurer takes care of the holy sanctuary ... and to [ ]a, daughter of [ ]milonicus, his virtuous wife ... for his happiness(?) dedicated ... H C"
Unfortunately, we can't tell who the subject of either verb (CURAT or DEDIT) is. The inscription could pertain to a Mithraeum, or conceivably to a Christian chapel.
Again, as with your Greek inscription, we're eager to know where you saw it. Or is it a secret? (-;
